If someone joins your forum using a proxy server is it always detectible? Is it possible for use of a proxy to be undetectible?

What about a spammer or troll using someone else's legitimate ip adress? Is that detectible?

Do "false positives" occur? Is it possible that someone could be wrongly viewed as using a proxy when they are not?

I read something about aol users being provided with proxies for legitimate purposes of hiding from stalkers, cyber bullies and other bad folks. I myself believe this can be a legitimate solution -- something used in good faith.

I bring this up because a member on our forum has been very vocal about our moderators strictly banning proxy users. As if it is the "end all be all," slam dunk solution to problems.

any opinions and/or articles on this would be appreciated

thanks for reading